Process Parameter Optimization and Microstructure of Hot Wire TIG Overlay Welding Inconel 625 on AISI 4130

Literature Overview

This paper published in Hot Working Technology (Vol. 44, No. 23, 2015, pp. 227–230) by Guo Longlong and colleagues from Southwest Petroleum University and Chongqing Xintai Machinery Co., Ltd. addresses a specific industrial need: improving corrosion resistance of throttling valves in acid gas fields by overlay welding Inconel 625 onto AISI 4130 steel substrates using hot wire TIG (HWT) welding. The research is funded by the Southwest Petroleum University Graduate Innovation Fund (CX2014BY05).

Process Selection Rationale

The selection of hot wire TIG for this application is technically well-motivated:

• Inconel 625 offers exceptional resistance to chloride stress corrosion cracking, sulfuric acid corrosion, and high-temperature oxidation—critical for acid gas field service.
• AISI 4130 (medium carbon alloy steel) provides the mechanical strength required for valve body pressure containment.
• Hot wire TIG enables high deposition rates (2–3× conventional TIG) with excellent weld quality, minimal dilution, and good process control—essential for dissimilar metal overlay applications.

Orthogonal Experimental Design

The authors employed orthogonal experimental design (Taguchi method) to optimize process parameters, with two response indicators:

The process parameters investigated likely included welding current, travel speed, wire feed rate, and hot wire current, though the paper focuses on the optimization results rather than detailed parameter ranges.

Microstructural and Mechanical Characterization

The optimized process produced overlay deposits with the following characteristics:

• Microstructure: Columnar austenite grains, which is the expected equilibrium structure for Inconel 625 at welding cooling rates.
• Phase composition: Confirmed by XRD and EDS to be predominantly γ-(Ni,Fe) austenite with Cr, Mo, and Nb in solid solution.
• Hardness: Approximately 230 HV, relatively uniform across the overlay thickness.
• Dilution: Controlled to acceptable levels, ensuring the overlay retains its corrosion-resistant properties.
• Defect-free weld: No porosity, cracking, or lack of fusion observed under the optimized parameters.

Critical Technical Considerations

Several engineering issues merit attention in translating this research to production:

1. Dilution control is paramount. Inconel 625's corrosion resistance depends on maintaining ≥55% Ni and ≥20% Cr in the overlay. Excessive dilution with carbon steel substrate degrades these properties rapidly. The hot wire TIG process offers better dilution control than GTAW or GMAW for this application.
2. Columnar grain structure in the overlay may create preferential crack paths parallel to the weld axis. In corrosive service, intergranular corrosion along columnar grain boundaries could be a concern. Post-weld solution treatment may be beneficial for grain refinement.
3. Thermal cycling effects during repeated welding passes can cause sensitization (δ-phase formation) in Inconel 625, particularly at interpass temperatures above 150°C. Strict interpass temperature control is essential.
4. Residual stress management is critical given the significant coefficient of thermal expansion mismatch between Inconel 625 (~13.3×10⁻⁶/K) and AISI 4130 (~12.3×10⁻⁶/K), combined with the high restraint of valve components.

Engineering Application Context

For acid gas field throttling valves, the corrosion challenge is primarily chloride-induced stress corrosion cracking and sulfuric acid pitting. The Inconel 625 overlay provides a robust solution, but the process parameters must be carefully controlled to ensure:

• Minimum overlay thickness of 3–5 mm for adequate corrosion allowance
• Dilution ratio below 30% to maintain corrosion resistance
• No micro-cracking in the overlay (particularly important for SCC resistance)
• Smooth surface finish to minimize crevice corrosion initiation sites

Study Insights

This research demonstrates that hot wire TIG is a technically viable and practical method for dissimilar metal overlay welding in pressure vessel and valve applications. The orthogonal experimental approach provides a systematic framework for parameter optimization that can be adapted to different substrate-alloy combinations. The relatively low hardness of the Inconel 625 overlay (230 HV) compared to the substrate is acceptable since the overlay serves a corrosion protection function rather than a wear-resistant one. Engineers should note that the successful application of this technique depends heavily on strict process discipline, particularly regarding interpass temperature control and dilution management.